[policy, no patch] Determine a best practice for providing BC for internal paths that change between minor releases

Let's call this a documentation issue, not "other".

Maybe the end result should be a new section on Drupal deprecation policy → , or a new sibling page.

Now that 🐛 Create a redirect for the new Block types path Fixed is fixed, we can start drafting the policy.

The remaining tasks has, "Ticket out". That does that mean?

Internal path

Add a route for the deprecated path. Name the route the same at the original adding ".bc" to the end. Set the path to the deprecated path. Add a controller, naming it to identify it as a redirect. Create the controller. The controller includes the @trigger_error('...', E_USER_DEPRECATED) and the necessary logic to perform the redirect.

For example:

# @todo Deprecate this route once
#   https://www.drupal.org/project/drupal/issues/3159210 is fixed, or remove
#   it in Drupal 11.
# @see https://www.drupal.org/node/3320855
entity.block_content_type.collection.bc:
  path: '/admin/structure/block/block-content/types'
  defaults:
    _controller: '\Drupal\block_content\Controller\BlockContentController::blockContentTypeRedirect'
  options:
    _admin_route: TRUE
  requirements:
    _permission: 'administer blocks'

  /**
   * Provides a redirect to the list of custom block types.
   *
   * @return \Symfony\Component\HttpFoundation\RedirectResponse
   *
   * @deprecated in drupal:10.1.0 and is removed from drupal:11.0.0. Use
   *   /admin/structure/block-content directly instead of
   *   /admin/structure/block/block-content/types.
   *
   * @see https://www.drupal.org/node/3320855
   */
  public function blockContentTypeRedirect(): RedirectResponse {
    @trigger_error('The path /admin/structure/block/block-content/types is deprecated in drupal:10.1.0 and is removed from drupal:11.0.0. Use /admin/structure/block-content. See https://www.drupal.org/node/3320855.', E_USER_DEPRECATED);
    $route = 'entity.block_content_type.collection';
    $params = [
      '%old_path' => Url::fromRoute("$route.bc")->toString(),
      '%new_path' => Url::fromRoute($route)->toString(),
      '%change_record' => 'https://www.drupal.org/node/3320855',
    ];
    $warning_message = $this->t('You have been redirected from %old_path. Update links, shortcuts, and bookmarks to use %new_path.', $params);
    $this->messenger()->addWarning($warning_message);
    $this->getLogger('block_content')->warning('A user was redirected from %old_path to %new_path. This redirect will be removed in a future version of Drupal. Update links, shortcuts, and bookmarks to use %new_path. See %change_record for more information.', $params);

    return $this->redirect($route, [], [], 301);
  }